1) what is/are an impressive, yet easily mass produced, cocktail ideal for an occassion where I might be hosting a "cocktail party" for 8-16 people and I want to make something in bulk, but impressive. Maybe do two or three recipes throughout the night

1) Make a punch! They're designed to sit around a little, just make sure you use a big block of ice to chill it. I'm partial to Fish House punch, but there are some great recipes out there. Also an excellent excuse to buy David Wondrich's book on punch.

Regarding the eggnog recipe: After you add the whites back in, does the eggnog become perishable? I want to make eggnog using this recipe, but it's unlikely we will drink it all immediately. Will adding the egg whites back in and then storing it have a negative impact on the taste/texture?

Most of the clear on clear stuff (Martini, Negroni, Manhattan) works pretty well in bulk. Just remember to add some water (10-20%) since they won't be diluted by shaking. Cut a bunch of citrus zest for garnish and you can easily serve one but still put on a tiny show of expressing the oil and garnishing the drink.

Punches are a good place to start, but you can put your own twist on a sour and prebatch that pretty easily/quickly (just make sure you have a juicer for all that citrus!). Cool sour combinations that make use of seasonal ingredients could be pear and rosemary, jalapeño/pepper and grapefruit, and apple with cinnamon. As jcfowler mentioned, be sure to add water (I like to stick with 25% because people will be self-serving and therefore consuming more than they would with shaken cocktails)!